Modern vehicles with internal combustion engines fueled by gasoline typically have a vapor control system that directs fuel vapor from a fuel tank vapor space to a carbon canister, rather than allowing the vapor to vent during refueling or during periods in which the fuel tank temperature rises. The canister is periodically purged so that the vapors are burned by the vehicle engine. If such a vehicle is parked on a grade for a prolonged period, liquid fuel may pass through the vapor vent valve to the canister, decreasing the performance ability of the canister.